This invention relates generally to a closed cycle cryogenic refrigeration system, and more particularly concerns refrigerant throttling devices having automatically variable flow areas.
FIG. 1 is a schematic of a closed cycle cryogenic refrigeration system 10 that was described in a paper, 80 K Closed Cycle Throttle Refrigerator, presented at the International Cryocooler Conference on Jun. 29, 1994, this paper being incorporated herein by reference. The refrigeration system 10 utilizes an oil lubricated compressor 11 connected at its discharge or high pressure end to an aftercooler 12 having its outlet connected to an oil separator 14. Oil from the oil separator 14 flows by way of a metering device 19 to the low side of the compressor 11.
Refrigerant from the oil separator 14 flows through an adsorber 15 and connects by a first line 16 to the high pressure inlet side of a regenerative heat exchanger 17. The adsorber 15 removes moisture from the refrigerant stream that may be present in the system and its components, as manufactured. The high pressure outlet of the regenerative heat exchanger 17 connects to the inlet of an adjustable throttle valve 18. An evaporator 21 connects between the discharge side of the throttle valve 18 and the low pressure side of the heat exchanger 17. The discharge end of the regenerative heat exchanger 17 connects by a second connecting line 20 to the low pressure inlet of the compressor 11 to complete the cycle.
The oil separator 14 is used downstream of the compressor 11 so as to remove liquid oil, pumped by the compressor as a lubricant, from the refrigerant flow stream that passes ultimately through the evaporator 21. Excess amounts of oil in the evaporator 21 affect heat transfer in the evaporator and would diminish the capacity of the cycle.
In order to minimize cool-down time at the evaporator 21, it is necessary to start with a large orifice opening in the throttle valve 18 and to continuously close this valve in order to maintain a nearly constant return pressure to the compressor of about 0.3 Mpa when cooling from start-up at room ambient conditions to a steady-state cryogenic operating temperature. For example, a needle valve serving as the throttle valve 18 may be set at 15 turns open at the start of cool-down and be set at only two turns open at the end of cool-down.
FIG. 2, taken from the above referenced paper, illustrates cool-down in a system with a fixed valve setting. However, cool-down would be faster if the orifice area of the throttle valve 18 were large and then was reduced during cool-down. In FIG. 2, T represents evaporator temperature; Ph is compressor discharge pressure, and P1 is the low pressure at the inlet to the compressor 11.
After cool-down, the orifice area of the valve 18 may be adjusted to establish a desired operating temperature in a range of about 80 to 100 K for a refrigerant mixture as described in the paper.
FIG. 3, also taken from the above-referenced paper, shows the relationships between heat load in the evaporator and evaporator temperature for different settings of the valve 18. The curve on the left side of FIG. 3 is for the smallest orifice setting during test and the curve on the right side of the graph is for the largest orifice used during the test. Reducing the orifice area in the valve 18 results in a lower flow rate of refrigerant that causes a reduction in the return pressure to the compressor 11. This lower pressure in turn reduces the temperature at which a mixed refrigerant boils. The lower flow rate also reduces the maximum rate of refrigeration.
A problem arises in that it is difficult to adjust the throttle valve 18 using a remote handle 22 that is located outside of the enclosure of the cryostat 23. The external handle 22 is a source of in-leakage of heat to the system and precise adjustment is frequently not repeatable. The result of an adjustment may not be immediately apparent, and there may be an undesirable lag between the adjustment and an intended result. As a result, a desirable smooth transition in temperatures upon making an adjustment is not always achieved.
A number of automatic control mechanisms have been developed for open cycle Joule-Thomson coolers (JT) that might be considered for a closed cycled application. However, important differences exist between the open circuit type system and the closed cycle system. Conventional open cycle JT coolers operate from bottles of high pressure gas, e.g. nitrogen at 40 Mpa (6,000 psig). This gas is throttled to produce a low temperature at lower pressure. The low temperature gas, partially liquified by throttling, passes through an evaporator and is then vented to atmospheric ambient. A back-pressure valve at the discharge generally determines the evaporator temperature.
Control mechanisms for JT coolers have also been developed to provide high flow rate for fast cool-down, followed by throttled flow after cool-down to reduce steady-state gas consumption in the open circuit systems. As stated, operating temperature is set primarily by the atmospheric vent pressure. When the orifice area remains constant during cool-down, the flow rate of refrigerant increases proportionally to T.sup.-1/2, a factor of 1.7 when the temperature changes from 300 K (room ambient) to 100 K. Gas always enters the restrictor valve, and the gas temperature is typically 20 K to 60 K warmer than the fluid that exits the valve.
Then, after cool-down, the valve flow area is typically reduced to 1% to 3% of its initial area so as to reduce the gas consumption. Cool-down time has been found to be minimum by having a valve that remains essentially wide open during cool-down to approximately 100 K. Then, the valve flow area is reduced to reduce gas flow rate.
FIG. 4 presents a comparison of the relative valve flow areas that are preferred to optimize cool-down time for typical open cycle JT cryostats and closed refrigeration cycle cryostats. For the closed cycle, cool-down time is minimized if the valve area is reduced from a maximum during cool-down. At the start of cool-down 100% gaseous refrigerant flows through the valve, while at the end of cool-down, refrigerant is either entirely or nearly 100% liquid. This change in density from gas to liquid requires a large change in valve area to keep the mass flow rate nearly constant.
By the time the evaporator temperature is at 100 K, the orifice area has been reduced in excess of 90% from its original value in the closed cycle. It is then usually necessary to further reduce the valve area to reach the desired operating temperature for steady-state.
As illustrated in FIG. 4, the change in area that is required for preferred cool-down in an open cycle (JT) is in the order to 50-60%. Then, a steady-state setting is made that is very similar to the setting in the closed cycle.
As stated, it is the pressure held in the evaporator before the refrigerant vents to atmosphere in an open cycle that determines the evaporator temperature. Varying the orifice in the JT control mechanisms controls the refrigerant flow rate in steady-state operation but does not control the temperature. To the contrary, in a closed cycle, when the orifice is adjusted it is primarily to reset the temperature in the evaporator and not the flow rate.
Thus, in an automatic open circuit JT cryostat, an increase in evaporator temperature causes the throttle valve to open and increase the flow rate. This increases the refrigeration effect and brings the temperature back to a desired level. In a closed cycle refrigerator, an increase in temperature at the evaporator, if accompanied by a refrigerant flow increase, would cause the temperature to increase even more as the compressor would operate at a higher inlet pressure to accommodate an increased mass flow of refrigerant. Therefore, in a closed cycle refrigeration system, for fast cool-down, the control mechanism, i.e., the adjustable throttle device, must start with a large orifice and reduce the flow rate during cool-down. Then the orifice must be set to a fixed position to maintain a nearly constant flow rate for steady-state operation of the system.
Other factors being substantially constant, a higher return pressure at the compressor inlet translates into a higher mass flow of refrigerant that results in a higher cooling rate. Therefore, a flow restrictor with a large orifice at the commencement of cool-down allows for a higher return pressure at the compressor than would exist if the orifice were preset, fixedly, to satisfy the desired steady-state conditions. Thereby, a valve that reduces flow area during cool-down provides a higher average return pressure at the compressor during cool-down for a given gas mixture, and cool-down will be faster than would be provided with a fixed restrictor.
What is needed is an automatic throttling device that has a continuously variable flow area during cool-down so as to minimize cool-down time, and holds a preset minimum flow area that will maintain desired operating temperatures after cool-down.